A few years back, I had a nasty bout of food poisoning while travelling, which made me lactose-intolerant for several years. Sounds like this may be more common than you'd think, based on this article:

If you haven’t heard of Whole30, some information: It’s a month-long eating plan that aims to help followers hit “the reset button with your health, habits, and relationship with food.” For 30 days, you cut out soy, legumes, grains, sugars, alcohol, and, of course, dairy. [....]

When you reach the end of the Whole30, you’re supposed to add the forbidden food groups back into your diet one at a time. The goal is to figure out which foods are making you feel sluggish, bloated, or just generally not great, so you can ostensibly keep on avoiding them forever.

I didn’t do that part. I just jumped right back into eating what I wanted — but suddenly nothing was the same. That first bowl of ice cream I’d been looking forward to for weeks was quickly followed by sharp stomach pains and what can best be described (grossly, but accurately) as bubble gut. [....]

The good news, according to gastrointestinal specialist Kim Barrett, a professor of medicine at the University of California, San Diego, is that I’m not crazy. The bad news is that dairy no longer agrees with my body’s biology. Turns out, it is possible to suddenly make yourself lactose intolerant.

“To some extent, our ability to handle lactose is a use-it-or-lose-it phenomenon,” Barrett says. The body digests lactose — a disaccharide — by using lactase, an enzyme in the small intestine, to break it down into the monosaccharides glucose and galactose, which can then be absorbed. “If you don’t have the [lactose] substrate in the diet, you start to reduce the synthesis of the lactase enzyme to digest it,” Barrett explains. “After a period of completely excluding lactose from the diet, you may not have any of those digestive enzymes present.”

The data clearly indicated that a nocebo effect, the same reaction that prompts some people to get sick from wind turbines and wireless internet, was at work here. Patients reported gastrointestinal distress without any apparent physical cause. Gluten wasn't the culprit; the cause was likely psychological. Participants expected the diets to make them sick, and so they did.

“Slim by Chocolate!” the headlines blared. A team of German researchers had found that people on a low-carb diet lost weight 10 percent faster if they ate a chocolate bar every day. It made the front page of Bild, Europe’s largest daily newspaper, just beneath their update about the Germanwings crash. From there, it ricocheted around the internet and beyond, making news in more than 20 countries and half a dozen languages. It was discussed on television news shows. It appeared in glossy print, most recently in the June issue of Shape magazine (“Why You Must Eat Chocolate Daily”, page 128). Not only does chocolate accelerate weight loss, the study found, but it leads to healthier cholesterol levels and overall increased well-being. The Bild story quotes the study’s lead author, Johannes Bohannon, Ph.D., research director of the Institute of Diet and Health: “The best part is you can buy chocolate everywhere.”

I am Johannes Bohannon, Ph.D. Well, actually my name is John, and I’m a journalist. I do have a Ph.D., but it’s in the molecular biology of bacteria, not humans. The Institute of Diet and Health? That’s nothing more than a website. Other than those fibs, the study was 100 percent authentic. My colleagues and I recruited actual human subjects in Germany. We ran an actual clinical trial, with subjects randomly assigned to different diet regimes. And the statistically significant benefits of chocolate that we reported are based on the actual data. It was, in fact, a fairly typical study for the field of diet research. Which is to say: It was terrible science. The results are meaningless, and the health claims that the media blasted out to millions of people around the world are utterly unfounded.

Interesting bit: the online commenters commenting on the published stories quickly saw through the bullshit. Why can't the churnalising journos do that?

Being paleo is like paying a stupidity tax. Again, it’s not you who is stupid, but the diet sure is, because it lets you drink paleo coffee while putting paleo butter and paleo syrup on your paleo waffles before you drive your paleo minivan to the paleo office to sit in your paleo cube and do spreadsheets on your paleo computer. See, the paleo diet made up a bunch of silly rules on how we allegedly ate, and then goes and twists them all to hell in the name of selling you a crappy, overpriced product. That is scientology-level stupid.